1. Field of the Invention
The present invention relates to an Echelle diffraction grating, an excimer laser using the same, and their manufacturing methods.
2. Description of the Related Art
A diffraction grating used for the wavelength selection (or narrow band) in an excimer laser serves as an optical element of a laser resonator, and thus needs a high diffraction efficiency and durability for a long-term service. One applicable diffraction grating is an Echelle diffraction grating (blazed diffraction grating) that has an asymmetrical triangular shape on a grating section.
For example, Japanese Patent No. 4,549,019 proposes a method for protecting an Al reflective film surface from being oxidized and deformed, by layering a protective film containing Al2O3 on the reflective film of each grating in the reflective Echelle diffraction grating. In order to propose a method of realizing a high initial value of the diffraction efficiency in the laser installation, U.S. Patent Application Publication No. 2009/0027776 adjusts a thickness of a protective film and U.S. Patent Application Publication No. 2010/0328775 adjusts a groove shape of a diffraction grating.
It is found that a ratio of the diffraction efficiency (orientation ratio) between the blazed order and an order lower by one than the blazed order after a laser beam is irradiated onto the Echelle diffraction grating is different from that before the laser irradiation. This difference means a deformation of a grating groove, and causes the diffraction efficiency of the blazed order to lower. The methods of U.S. Patent Application Publication Nos. 2009/0027776 and 2010/0328775 are insufficient as solutions for this problem.